Receptors, hormones, enzymes, ion channels and structural components of the cell are created by protein synthesis. Synthesis alone is insufficient for proper function. For a cell to operate effectively, its components must be correctly compartmentalized. Proteins containing mutations that lead to mis-folding of the protein and resulting aggregation of the protein or mis-routing of the protein can result in disease due to loss of protein function, creation of toxic functions or interference with normal functions of other molecules. For example, mutations in amyloid precursor protein result in aggregation of the amyloid protein, which can result in Alzheimer's disease, while mutations in the gonadotropin-releasing hormone receptor (GnRHR) can result in an inability of the GnRHR to properly traffic to the cell surface, resulting in hypogonadotropic hypogonadism (HH).
Several approaches have been applied to salvage defective proteins. Among these are genetic approaches, such as increased receptor expression to produce larger numbers of receptors (Cheng et al., Am. J. Physiol. 268:L615-24, 1995; Schülein et al., J. Biol. Chem. 276:8384-92, 2001; Maya-Nùñez et al., J. Clin. Endocrinol. Metab. 87:2144-9, 2002). However, increasing the number of properly folded or properly trafficked proteins in a cell by genetic approaches is not presently practical for in vivo use.
Other methods of correcting defective receptors include the use of non-specific protein stabilizing agents to stabilize extant molecules rendered incompetent by genetic defects, such as polyols and sugars (Back et al. Biochemistry 18:5291-6, 1979; Brown et al., J. Clin. Invest. 99:1432-44, 1997; Brown et al., Cell Stress Chaperones 1:117-24, 1996; Sato et al., J. Biol. Chem. 271:635-8, 1996; U.S. Pat. Nos. 6,541,195 and 6,270,954 to Welch et al.), or physical methods (Denning et al., Nature 358:761-4, 1992; Brown et al., J. Clin. Invest.; 99:1432-1444, 1997; Matsuda et al., J. Biol. Chem.; 274:34515-8, 1999; Zhou et al., J. Biol. Chem. 274:31123-6, 1999). However, previously disclosed protein stabilizing agents are non-specific, leading to many side effects, and the amount of agent needed to treat the defect would be toxic in vivo, which generally precludes their use in a clinical setting (Welch, Semin. Cell Dev. Biol. 15:31-8, 2004).
Therefore, there is a need for a method to restore function to mutant proteins that are mis-folded, such as mutant receptors, that is specific for the protein of interest and which can be used therapeutically in vivo. In addition, there is a need for methods that allow one to screen for therapeutically effective agents that can be used to treat subjects having diseases associated with mis-folded mutated proteins that result in protein aggregation or protein mis-routing in a cell.